Interaction of the molecular motor myosin with actin drives muscle contraction and various changes in cell shape and movements that are fundamental to the biology of nonmuscle eukaryotic cells. Essential to the function of myosin in nonmuscle cells is the regulation of its assembly into filaments and higher order structures such as the contractile ring, the assembly of which is spatially and temporally controlled. Little is known about the molecular basis of such spatial and temporal control but phosphorylation of the myosin molecule appears to play a key role. Light chain phosphorylation may regulate myosin filament assembly in some systems, but may primarily control the motor function of myosin. Heavy chain phosphorylation may be primarily involved in regulation of myosin filament assembly. The specific aim of this proposal is to study the roles of myosin phosphorylation in Dictyostelium, an organism that allows the convergence of biochemical, molecular genetic, and physiological approaches. It therefore offers a unique opportunity to investigate the molecular basis of how a cell moves, divides, and changes shape in response to cellular and developmental signals. The experimental plan for the next five year period can be divided into three parts: 1. Biochemical, structural, and molecular genetic studies on the kinases and phosphatases that are responsible for the control of Dictyostelium myosin phosphorylations. 2. Biochemical and structural studies on the effects of these phosphorylations on the functions of purified myosin. 3. Characterization of the effects of phosphorylation on the in vivo behavior and function of the myosin. The latter is made possible by recent applications of sophisticated molecular genetic techniques to Dictyostelium.